Fusion Power: From Toroidal Pinch to Tokamak

What is Nuclear Fusion?

Nuclear fusion refers to the reaction where two atomic nuclei collide and transform into a single heavier nucleus. Fundamentally, atomic nuclei have a positive charge due to the protons inside, so when two nuclei approach each other, they repel each other due to electrical repulsion. However, if the nuclei are heated to extremely high temperatures, their kinetic energy can overcome the electrical repulsion, allowing the two nuclei to collide. Once the two nuclei approach sufficiently close, the strong nuclear force takes effect, causing them to combine into a single nucleus.

In the late 1920s, when it became known that the energy source of stars was nuclear fusion and it could be physically explained, discussions began on whether nuclear fusion could be used for the benefit of humanity. Not long after the end of World War II, the idea of controlling and utilizing fusion energy was seriously considered, and research began at the University of Liverpool, Oxford University, and the University of London in the UK.

Break-even Point and Ignition Condition

One of the most fundamental issues for fusion power is that the energy produced from the fusion reaction must be greater than the initial input energy. In the DT reaction, alpha particles and neutrons are produced, with 20% of the energy released by fusion carried by alpha particles and 80% by neutrons. The energy of the alpha particles is used to heat the plasma, while the energy of the neutrons is converted into electrical energy. Initially, external energy must be applied to raise the plasma temperature, but once the fusion reaction rate increases sufficiently, the plasma can be heated solely by the energy of the alpha particles, allowing the fusion reaction to sustain itself. This point is called ignition, and it occurs when $nT\tau_{E} > 3 \times 10^{21} m^{-3} keVs$, or $\text{plasma pressure}(P) \times \text{energy confinement time}(\tau_{E}) > 5$ in the temperature range of 10-20keV (approximately 100-200 million K).

Toroidal Pinch

In 1946, Peter Thonemann conducted research on confining plasma in a torus using the pinch effect at the Clarendon Laboratory of Oxford University.

As shown in the figure, when current is passed through the plasma, a magnetic field forms around the current, and due to the interaction between the current and the magnetic field, a force acts inward. Theoretically, if the current is large enough, the pinch effect can prevent the plasma from touching the wall. However, experimental results showed that this method was very unstable, and therefore it is hardly being researched now.

Stellarator

In the early 1950s, astrophysicist Lyman Spitzer at Princeton University invented a new plasma confinement device and named it the stellarator. Unlike the toroidal pinch where the magnetic field is created by the current flowing through the plasma itself, in the stellarator, the magnetic field is formed only by external coils. The stellarator has the advantage of being able to maintain plasma stably for long periods, and it is still recognized as having sufficient potential value to be actually applied to fusion power plants. Research is still actively ongoing.

Tokamak (toroidalnaya karmera magnitnaya katushka)

By the 1960s, fusion research had entered a period of stagnation. Around this time, the Kurchatov Institute in Moscow first devised the tokamak, finding a breakthrough. When the results of the tokamak were presented at an academic conference in 1968, most countries shifted their research direction towards tokamaks, and it has become the most promising magnetic confinement method currently. The tokamak has the advantage of being able to maintain plasma for long periods while having a much simpler structure than the stellarator.

Large Tokamak Devices and the ITER Project

Since the 1970s, large-scale tokamak devices have been built to get closer to actual fusion power generation. Notable examples include the European Union’s JET, Princeton’s TFTR in the United States, and Japan’s JT-60U. By consistently conducting research to increase output in these large tokamaks based on data obtained from small-scale experimental devices, they have almost reached the break-even point. Currently, to make a final check on the possibility of fusion power, China, the European Union, India, Japan, Korea, Russia, and the United States are cooperating on the ITER project, humanity’s largest international joint project.

References

• Khatri, G.. (2010). Toroidal Equilibrium Feedback Control at EXTRAP T2R.
• Garry McCracken, Peter Stott, 핵융합: 우주의 에너지, 유창모 외 2명 번역, 북스힐